22 Mar 2010: Analysis

The Secret of Sea Level Rise:
It Will Vary Greatly by Region

As the world warms, sea levels could easily rise three to six feet this century. But increases will vary widely by region, with prevailing winds, powerful ocean currents, and even the gravitational pull of the polar ice sheets determining whether some coastal areas will be inundated while others stay dry.

by michael d. lemonick

For at least two decades now, climate scientists have been telling us that CO2 and other human-generated greenhouse gases are warming the planet, and that if we keep burning fossil fuels the trend will continue. Recent projections suggest a global average warming of perhaps 3 to 4 degrees C, or 5.4 to 7 degrees F, by the end of this century.

But those same scientists have also been reminding us consistently that this is just an average. Thanks to all sorts of regional factors — changes in vegetation, for example, or ice cover, or prevailing winds — some areas are likely to warm more than that, while others should warm less.

What’s true for temperature, it turns out, is also true for another frequently invoked consequence of global warming. Sea level, according to the best current projections, could rise by about a meter by 2100, in large part due to melting of the Greenland and West Antarctic ice sheets. But that figure, too, is just a global average. In some places — Scotland, Iceland, and Alaska for example — it could be significantly less in the centuries to come. In others, like much of the eastern United States, it could be significantly more.

And among the most powerful influences on regional sea level is a surprising force: the massive polar ice sheets and their gravitational pull, which will lessen as the ice caps melt and shrink, with profoundly different effects on sea level in various parts of the globe.

If the idea of local differences in sea level comes as a surprise, it’s probably because the experts themselves are only now beginning to fully realize what might cause such differences, and how significant they might be. One
Prevailing winds can push water consistently toward the land or keep it at bay.
factor, which they’ve have been aware of for decades, is that the land is actually rising in some places, including northern Canada and Scandinavia, which are still recovering from the crushing weight of the Ice Age glaciers that melted 10,000 years ago. That makes sea-level increases less than the global average would suggest, since these land areas are rising a few millimeters a year.

Around the periphery of where the glaciers sat, by contrast — places like Chesapeake Bay and the south of England — the land was actually squeezed upward during the Ice Age by the downward pressure nearby. The resulting “glacial forebulge” has been sinking back ever since, also at an average rate of a few millimeters a year, so sea level rise is greater than average in these regions.

And in some coastal areas — most notably along the Gulf of Mexico in Louisiana — the land is falling as well: Thanks to massive oil and gas extraction, the continental shelf is collapsing like a deflated balloon. “The rate of subsidence measured at Grand Isle, Louisiana,” says Rui Ponte, of the private consulting firm Atmospheric and Environmental Research, Inc, “is almost 10 millimeters per year, compared with two or three in other areas.” That’s especially problematic for a city like New Orleans, which already lies partly below sea level.

Ponte said that these local instances of rebound or subsidence will subtract or add a couple of inches to the global increase in sea level over the next century, depending on the region.

A bigger effect will come from changes in prevailing winds, which can push water consistently toward the land or keep it at bay. The trade winds that blow west across the tropical Pacific, for example, move water in the same direction, boosting average sea levels by as much as 24 inches on the western side of the ocean — in places such as the Philippines — compared with those in northern South America. If those winds shift with climate change, so would local sea levels.

Ocean currents can also create significant local effects. During preparations for the Intergovernmental Panel on Climate Change’s Second Assessment Report back in the mid-1990s, Ronald Stouffer — a climate modeler at the U.S. Geophysical Fluid Dynamics Laboratory in Princeton, NJ — and several colleagues were comparing projections of regional sea-level rise from different models.

“They were splattered all over the place,” he recalls, “and the differences had no rhyme or reason. We speculated that they had to do with differences in how the models treated changes in the prevailing winds.”

But a little more than a year ago, Jianjun Yin, now at Florida State University, suggested that it might be something else: a weakening
The polar ice caps keep sea level higher for thousands of kilometers around both land masses.
of the “overturning” that drives major ocean currents. In the Atlantic, it works like this: Warm surface water — the Gulf Stream — flows north and east until it reaches the area between the United Kingdom and Greenland, where it cools, thus becoming denser, and sinks. It flows south and west, deep below the surface. Eventually, it rises again, warms, and heads back north.

If any part of this flow is significantly interrupted, the current will slow. Global warming has the potential to do just that, in two ways. First, a warmer North Atlantic won’t let the surface water cool so easily, interfering with its tendency to sink. Second, fresh water from Greenland’s shrinking ice cap dilutes the surrounding waters; since fresh water is less dense than salty water, there’s a further impediment to sinking.

Since the Gulf Stream warms northern Europe, the slowing could cool that part of the world. But the slowing would also force water to pile up behind what amounts to a partial blockage of the overturning current. That could force sea level along the U.S. coast to rise another 8 or so inches over the next century beyond the global average, given a medium-emissions scenario.

When he first heard about this idea, says Stouffer, “it was one of those ‘duh’ moments for me. I said, ‘Yeah, that makes sense.’” He ended up co-authoring a paper that appeared in Nature Geoscience last March, laying out the argument.

Then, however, Stouffer experienced another “duh” moment. “I’m somewhat embarrassed by that paper,” he says, “because here we were focused on this relatively little problem, and there’s this great big gorilla in the room, and I missed it. But I had a lot of company.” (This last point is crucial: Stouffer is among the most experienced and respected modelers in the world, so a “duh” moment for him means the surprise is widespread.)

The gorilla Stouffer refers to — an effect so large that it overwhelms the others — is something called the geoid. It’s an imaginary surface that maps
If the West Antarctic Ice Sheet melts and loses mass, its pull on the surrounding ocean will lessen.
the strength of Earth’s gravitational field, and it’s as bumpy as the surface of the actual planet. Orbiting satellites don’t move around the Earth in perfect circles, or even perfect ellipses; their height changes when they go over the extra gravity exerted by a mountain range, and changes again when they orbit over a valley.

And because water is a liquid, the surface of the sea is also warped to follow the contours of the geoid. The extra gravitational attraction of an undersea mountain range pulls water toward it, creating a literal, permanent bump on the surface of the sea, while the deficit of gravity near an undersea valley creates a depression in the water up above.

The same sort of thing happens when there’s an excess of mass on land that lies near the ocean. A coastal mountain range pulls the water in its direction, raising sea level nearby. So do the massive icecaps that smother Greenland and Antarctica. Indeed, Antarctica’s polar ice sheet is so massive that it is three miles thick in places and covers an area one-and-one-half times the size of the United States, including Alaska.

These polar ice caps are Stouffer’s gorillas. They keep sea level higher than it would otherwise be for thousands of kilometers around both land masses, and correspondingly lower elsewhere.

If the polar ice sheets shrink, though — as they’re currently doing, especially in Grenland and West Antarctica — their gravitational pull weakens and so does their hold on the surrounding water. About a year ago, Jerry Mitrovica, a geophysicist who teaches an entire course on sea level at Harvard, co-authored a paper in Science that laid out what would likely happen if the West Antarctic ice sheet, the smaller of the two sheets that cover the Antarctic continent, were to melt. (Like a complete shutdown of the Gulf Stream, this is not considered likely anytime soon. But recent satellite measurements have shown that glaciers that drain the ice sheet have begun moving faster toward the sea).

If you simply spread the resulting increase in sea level evenly around the world, it would amount to about 5 meters’ worth. But the ice sheet’s gravity is currently keeping sea level artificially low in the Northern Hemisphere, so if it disappeared, the actual increase along the U.S. mid-Atlantic coast would be more like 6.3 meters. In other words, as the West Antarctic Ice Sheet melts and loses mass, its pull on the surrounding ocean will lessen. Seas will drop around Antarctica and parts of the Southern Hemisphere, and that water will be displaced to more northerly areas, such as the east coast of the U.S.

Now that the gorilla has made its presence known, Stouffer is working with Mitrovica to understand its effects in greater detail. A joint paper, due out in a few months, will look into the gravitationally driven sea-level changes a melting Greenland could trigger. “The signal is so large,” says Stouffer, “that if you own beachfront property in Iceland, and all of the ice on Greenland melts and adds seven meters to average sea level, you end up with more beach. But in Hawaii, you get your seven meters of sea-level rise plus an extra two or three on top of that. It’s phenomenal to me that it matters that much.”

Mitrovica agrees.

“When I give talks about this, people don’t believe me,” says Mitrovica. He doesn’t blame them, either. “It’s just wacky when you think about it, completely counterintuitive,” he says. “But it’s true.”


How High Will Seas Rise?
Get Ready for Seven Feet

Rising Seas
As governments, businesses, and homeowners plan for the future, they should assume that the world’s oceans will rise by at least two meters — roughly seven feet — this century.
It’s even measurable, despite the fact that the melting of the ice sheets has barely begun. Even when you correct for other effects, says Mitrovica, you can still see that Europe’s sea level rise is less than you’d expect. “It’s profoundly puzzling,” he says, “until you realize you’re seeing the gravitational signal of Greenland melting.”

When he started looking at regional effects, Mitrovica recalls, some climate-change deniers were noting that sea-level rise was happening at different rates in different regions, arguing that this proved there was no global trend, and thus no global warming. That was already a bogus argument, but now that he and others have begun investigating the gorilla in the living room, it’s even more absurd. The science is so straightforward, he says, that “if you saw that sea level was rising uniformly around the world, it would be proof that the big ice sheets are not melting.”

POSTED ON 22 Mar 2010 IN Climate Climate Forests Policy & Politics North America North America 


How about plate tectonics? In the Cretaceous, there was a Western Seaway connecting the Gulf of Mexico to the Arctic Ocean. Thousands of meters of sediments were deposited, then eroded as this land was uplifted.

How do you factor out the effects of tectonics, or do you even want to bother? That is your real 8000 lb. gorilla.

Posted by Shoshin on 22 Mar 2010

Plate tectonics is a major factor when looking at earth's climate. The current configuration of continents with the bias towards the Northern Hemisphere and the South Pole surrounded by a continent which is in turn completely surrounded by ocean, along with the current balance between volcanic activity and continental weathering is the primary reason the earth is in an "icehouse" age.

For the purposes of climate modelling over tens and hundreds of years, these factors can be assumed to be in a steady state.

Posted by Anthony David on 22 Mar 2010

Tectonics matter on millions of years timescales, orbital changes matter on thousands to hundreds of thousands of years.... Read Earth's Climate: Past and Future if you want some background on that.

Posted by Robert on 22 Mar 2010

Sea level is currently rising at about 3.1 mm per year. That works out to about 282 mm or 11 inches by the end of the century or 12.2 inches per 100 years. 12.2 inches per 100 years is about half the average increase over the last 20,000 or so years.

The 2007 IPCC report estimated that sea level would increase between 7.5 and 23.2 inches by 2100. This article claims that it COULD easily go up by a meter (39 inches). In essence this is nothing but the usual scare tactics.

Posted by lomboz on 22 Mar 2010

RE: lomboz

You're only taking the current annual rise and multiplying it out to the end of the century, as if this rate of change is stable and not increasing. A valid projection needs to take in to account the increased future rate of change due to thermal expansion among other factors, which is dependent on various emission scenarios linked to warming.

The IPCC report was very conservative and decidedly did not take in to account any contribution from ice sheet collapse as ice sheet dynamics were viewed as too poorly understood at the time.

There is mounting observational evidence that ice sheets are indeed losing mass, which is what this article is focusing on. The meter rise you reference from the article is additionally being projected for SOME areas due to the disproportionate distribution of sea level rise from the diminished gravitational effects of the ice sheets themselves. I think the article is less scare tactics than a thoughtful assessment of another important facet of predicting future sea level rise.

Posted by Scott on 23 Mar 2010

I guess I am with lomboz and always try to balance the study, theorizing, and speculation with observation. Here's what we have observed - sea levels rising at a slower and slower rate over the last 7,000 years. Of course there's always debate about it as the GWart article suggests.

Posted by dbleader61 on 24 Mar 2010

"melting of the Greenland and West Antarctic ice sheet" What?

Where is there evidence of "melting" of the West Antarctic ice sheet. This statement defies both science and logic. Melting has a specific meaning. Let's use the language properly.

The West Antarctic Peninsula, a place that was once anomalously cold, is now anomalously warm. It is the ONLY place in the Antarctic that has shown any sign of "warming".

The peninsula acts like a foot extended in a Jacuzzi. One that catches uncomfortable vortexes of hot and cold from the South Oceans. It is no indicator of what is happening elsewhere on the continent.

However, the Antarctic sea ice extent is an indicator. It indicates the Antarctic is cooling down. For the last couple of years the Antarctic sea ice has set record highs.

Please let us separate hype and hysteria from science.

Posted by GregS on 24 Mar 2010

1. Ocean Current Shutdown IS a possibility soon -- even THIS YEAR -- it is true that 1 of the 2 causes: salinity changes -- has been downgraded but the present DOUBLE-EL-NINO may melt the ENTIRE Arctic Ocean off in 1 year. As the "driver" of currents is: to convey the excess Tropical Heat to the Cold Arctic ... a Warm Arctic would cause a Current Shutdown -- until it resumes in Winter. Until the 3-knot current gets water TO the Arctic 3 months later, we risk 300 mph winds taking up the slack.
(Open Water absorbs 4 times the Sunlight that Ice does so the North heats up REAL fast if the Surface is revealed).

2. You miss the "Word Trick" -- 'IF the West Antarctic melts off' -- this is NOT a projected trend.

"IF" specifies: ALL of it melting.
Melting of Miles-plus thick sheets took 1000s of years at the end of each Ice Age -- and will do the same ,again, especially so close to the Pole. The next Century's melting is only a few percent of the TOTAL West A. Sheet. So the 5-meter Case is only a theoretical exercise.

Posted by Charles Wilson on 24 Mar 2010

A bit of exaggeration here by your source regarding to the "duh" moment. The reaction of sea level to changes in gravity caused by melting ice caps is well known and has been modeled to explain past sea level rise. Thomas Cronin discussed it at length over 10 years ago in the chapter on sea level rise. I was aware of it and I'm a humble coastal wetlands biologist.


Re: the other comments. Sea level rise rates have recently accelerated and continue to do so. They are now at 3.3 mm/year and are still accelerating. They have doubled in just the last 20 years. The cause, melting ice in Greenland, Antarctica and the world's mountain glaciers, is well known. Antarctica is melting, not because of increased air temperatures though they have increased over most of Antarctica (west and east); but rather because increased wind speed (from both global warming and ozone depletion) is forcing more relative warm ocean water (just barely above freezing) underneath the continent's fringing ice shelves and is melting them and thus releasing the holding pressure on the glacial flow of the grounded ice sheets behind them.

Posted by Andy on 25 Mar 2010

Mr. Lemonick's article is truly thought provoking. Following the logic of the scientists concerning sea level variances attributed to gravitational forces of the earth, would it not stand to reason that the gravitational pull of the moon (the tides) causes all that additional water from melted ice sheets to move more dramatically as the earth rotates? In other words, have the scientists examined whether high tides will be higher and low tides lower as average sea level rises over the next 100 years?

Should such a hypothesis prove true, the impact of rising sea levels could have devastating effects to humans and other life on the planet as part of the impact from global warming.

Posted by BrianW on 25 Mar 2010

Sea level rise accelerating? Not. The rise has slowed over the last four years.
See http://wattsupwiththat.com/2010/02/1

Weakening of the overturning ocean current? NASA says no. See http://www.physorg.com/news188827980.html

This guy needs to deal more in facts and less in fright.

Posted by Willis Eschenbach on 27 Mar 2010

Yes, sea level rise did recently accelerate, but according to the latest figures from the argo buoy system,that aceleration finished a few years ago, and , puhleese , stop trying to suggest that antarctic sea ice is decreasing, or that antarctica is warming: the science is against you. Back to the aricle. Actually ,the money is on a sea level rise of 20cm [7 inches] over the next 100 years, which is the lower most of the IPCC projections. This would be totally consistent with the sea level rise over the last 5000 years. Prior to that ,of course, we had 125 meters of sea level rise over about 12000years. which created the pacific islands, the great barrier reef , and Sydney Harbour, amongst many other favourite places!

Posted by ian hilliar on 28 Mar 2010

There have been some unusually high tides of late (as in 2' higher than expected by NOAA):


It would be speculation to corrolate this with global warming/meltwater runoff/gravitational pull, but as they are searching for answers, maybe some should speculate thusly.

Posted by Everett Rowdy on 29 Mar 2010

Sea level rise per hundred years. Most of that rise happened at the end of the last glaciation 8000 to 18000 years ago. For the last 6000 years or so, while we've been multiplying with biblical enthusiasm and building our civilization, sea levels have been stable. The fact that sea level is now rising again at the rate of 1 foot per 100 years isn't the benign normality that your comment implies. This is something we haven't seen since the neandertalers disappeared.

Posted by Jim on 07 Apr 2010

A good article but then the deniers turn up in the comments to spout nonsense using their favorite tactic of accusing any science they deny to be "alarmist"

"Sea level rise accelerating? Not. The rise has slowed over the last four years."

The acceleration is over decades and has a good explaination for it's continuation. Four years is nowhere near enough to determine that such acceleration has stopped.

What's next, claiming global warming has stopped because temperatures fall a bit as we enter a La Nina? Yea that is probably next on the FUD list for the deniers.

Posted by cthulhu on 07 Jul 2010

"And in some coastal areas — most notably along the Gulf of Mexico in Louisiana — the land is falling as well: Thanks to massive oil and gas extraction, the continental shelf is collapsing like a deflated balloon."

Could this be mitigated by sequestering CO2 there? It is likely long shot, but is it even remotely realistic?

Posted by Mike on 14 Mar 2011

Your article is truly thought provoking. Following the logic of the scientists concerning sea level variances attributed to gravitational forces of the earth, would it not stand to reason that the gravitational pull of the moon (the tides) causes all that additional water from melted ice sheets to move more dramatically as the earth rotates? In other words, have the scientists examined whether high tides will be higher and low tides lower as average sea level rises over the next 100 years? Should such a hypothesis prove true, the impact of rising sea levels could have devastating effects to humans and other life on the planet as part of the impact from global warming.

Posted by ora on 07 Jun 2011

Comments have been closed on this feature.
michael d. lemonickABOUT THE AUTHOR
Michael D. Lemonick is the senior writer at Climate Central, a nonpartisan organization whose mission is to communicate climate science to the public. Prior to joining Climate Central, he was a senior writer at Time magazine, where he covered science and the environment for more than 20 years. He has also written four books on astronomical topics and has taught science journalism at Princeton University for the past decade. In other articles for Yale Environment 360, Lemonick has written about new evidence that makes the 2007 report by the Intergovernmental Panel on Climate Change already outdated and a recent report on the impacts of climate change in the United States.



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